Decomposition of materials within a landfill produces landfill gas which contains methane. In some instances, the methane concentration is sufficient to warrant recovery of the landfill gas so that the methane can be used as a fuel. To increase the concentration of methane, a landfill gas purification plant can be installed at the landfill.
The recovery of landfill gas requires a substantial investment for the construction of landfill gas recovery wells. Landfills are not homogeneous and do not produce landfill gas at the same rate at all locations within the landfill. Accordingly, the selection of each landfill gas well site is very important in order to maximize gas production and to minimize the number of wells which cannot be used or cannot be used for long periods due to inadequate production.
One method of locating landfill gas wells is described in Johnson et al. U.S. Pat. No. 4,026,355. Although this method has been used for many years, it would be desirable to have a lower cost method for locating wells at the best possible sites.
Landfill gas is produced by the landfill at a positive pressure. This positive pressure causes the landfill gas to flow upwardly to the upper surface of the landfill and vent to the atmosphere. Another way to locate sites for landfill gas wells is to measure the venting rate of the landfill gas at various regions of the landfill. These regions may be at the landfill's upper surface or, if desired, may be at any region below the upper surface of the landfill.
One approach to measuring landfill gas venting rates is to place a receiver on the landfill surface so that the venting landfill gas can flow into the receiver. A methanometer probe is provided in the receiver so that, based upon gas mixing considerations, the venting rate of the landfill gas can be calculated.
Unfortunately, the measurement of the landfill gas venting rate is plagued with problems, and heretofore the accurate measurement of the landfill gas venting rate has not been attained. For example, wind impinging on the base of the receiver at the interface with the landfill surface may cause a net inflow of air into the receiver and decrease the methane concentration. Wind impinging on any port of the receiver tends to push the gas within the receiver back into the landfill or out into the atmosphere at the landfill surface. Also, the methanometric approach requires that the gas inside be fully mixed or at least the mixing regime be well known, and neither is generally the case. In addition, the methanometric approach may pose a sensitivity problem due to the relatively high lower detection limit of commonly available field methanometers.
The single most important drawback to measuring the landfill gas venting rate is that the collection effeciency is very low. Collection efficiency is equal to the total volume of gas entering the receiver through its base area divided by the total amount of gas which vents over the same area in the absence of the receiver. In some laboratory testing of shallow receivers, collection efficiencies as low as 0.10 percent have been noted.